The conditioning of oxide surfaces or scale on metal surfaces, sometimes referred to as descaling, is desired with respect to the production of stainless steel and superalloy metal strips. While our discussion focuses primarily on metals in strip form, the applicability and value of our invention may be useful for conditioning oxide surfaces or scale in various shapes, geometries, or assemblies other than metal strip; it is not our intention to limit the benefit to only metal strip. Stainless steels are ferrous alloys containing more than about 10% chromium for the purpose of enhancing corrosion and oxidation resistance, and may also contain nickel, molybdenum, silicon, manganese, aluminum, carbide formers and other elements. Families of superalloys may contain nickel or cobalt as the predominant base element, and incorporate more exotic alloying elements, such as tungsten, titanium, niobium, and other elements. All of these base elements and additive elements have a high affinity for oxygen at high temperatures and form tenacious and chemically stable oxides which complicate their subsequent removal which is required prior to additional processing or sale.
Prior art descaling techniques for some grades of low alloy steels with very light scale include pickling of steel strip in mineral acid, such as sulfuric acid, hydrochloric acid, hydrofluoric acid, nitric acid, or mixtures thereof. However, often a mere acid pickle is insufficient in treating higher alloy steel strips. Conditioning of the scale before acid pickling may be required. Typical compositions used for scale conditioning are caustic mixtures of alkali metal hydroxides and alkali metal nitrates with various other additives such as alkali halides carbonates, and/or other oxidizing agents, often referred to as descaling or scale conditioning salts. A conventional technique for using such compositions is in a bath of fused anhydrous salt in a vessel at elevated temperatures, e.g. 427° C. (800° F.) to 538° C. (1000° F.), in which a metal object is first immersed, followed by a water rinse and acid pickle.